1. Technical Field
The present invention relates to barriers which allow air to flow from one side of the barrier to the other while inhibiting the passage of solid objects therethrough. More particularly, the present invention relates to ballistic and forced entry resistant barriers.
2. Background Information
Many applications exist where adequate ventilation of an enclosed area is imperative while maintaining a relatively high degree of forced entry and ballistic resistant integrity. For example, in security installations having a power substation or generator, it is important to provide adequate ventilation of the enclosed power source while minimizing the chances of successfully sabotaging the power source by access through the ventilation means.
The typical method of indicating ventilation efficiency is by reference to the percent Free Area, as defined by the Air Movement and Control Association (AMCA) Standard 500-83. Percent Free Area is the minimum cumulative open pathway area divided by the total area of barrier surface facing the air flow. While percent Free Area exhibited by architectural quality louvers (i.e., non ballistic) are typically 40 to 50 percent, the prior art ballistic barriers typically can achieve no more than about 20 to 33 percent Free Area, with many types only exhibiting about 50 to 10 percent Free Area.
Various configurations for bulletproof barriers are known in the art. For example, U.S. Pat. No. 1,953,708 issued to Fuller discloses a bulletproof structure for use in bank teller cages. In Fuller, the bulletproof barrier comprises a top portion of clear bulletproof glass and a bottom portion of bulletproof steel channels arranged in a staggered layer arrangement. The top portion includes means for ventilation which comprises two sheet of bulletproof glass, each sheet having openings which allow for ventilation. The openings are staggered with respect to each other so that a projectile cannot make its way through both openings. The Free Area area of the Fuller barrier is about 30%.
Other bulletproof barriers for bank tellers are also known, such as the baffle arrangement shown in U.S. Pat. No. 3,749,028 issued to Strobl. Strobl discloses a bulletproof baffle for positioning within a communication opening in a bulletproof window. The baffle has an X-shaped cross-section permitting the passage of sound waves but obstructing straight line axis for a bullet or the like. A major drawback to Strobl is that bullets are able to ricochet off the X-shaped cross-section, causing possible harm to one on the other side of the barrier. Additionally, Strobl has a very low Free Area range of about 15 to 25%.
One of the biggest problems with conventional prior art bulletproof structures is their lack of adequate ventilation channels. As stated previously, conventional ballistic barriers have a Free Area range between 20 to 33%. For comparison, typical prior art louvers which are not designed for ballistic protection tend towards a 40 to 50% Free Area range. Due to the relatively low Free Area ranges of conventional ballistic barriers, alternative ventilation systems are required for increased ventilation. Several ventilation system configurations are known in the art. For example, U.S. Pat. No. 1,218,684 issued to Mitchell discloses a ventilation device for a transom above a doorway. The device comprises two sets of staggered C-shaped plates which are movable relative to each other to permit an adjustable range of ventilation. Although Mitchell allows variable ventilation, the system is far from bulletproof, for the C-shaped plates allow projectiles a direct access through the ventilation system.
However, other venitlation systems have been developed which do not allow direct access through the ventilation system. For example, U.S. Pat. No. 2,424,219 issued to Black eliminates any direct access by a complex series of vanes and troughs designed to prevent sand, dust, and the like from passing through the ventilation means. Black's system has a Free Area of only about 25%. In addition, U.S. Pat. No. 547,356 issued to Pancoast discloses a system with two rows of C-shaped channels diametrically opposed so that their concave sides face each other. The channels are staggered so that a direct line of sight is eliminated.
The biggest drawback to incorporating an alternative and supplemental ventilation system is that of increased security risks. It may not be feasible to incorporate a supplemental ventilation system in the security area for reasons such as the unacceptable increase in security risks, additional costs, area constraints, undesired system complexity, or the like.